The dynamics of interaction between hemostatic cells (platelets) and inflammatory cells (PMN) provides a basis for understanding relevant pathophysiologies. Platelets may aid PMN participation at an inflammatory site and PMN may function in thromboregulation. Data in vitro on platelet-PMN interactions suggest that these cell interactions are of importance for both inflammation and hemostasis. We are investigating platelet-PMN interactions and the functional consequences of platelet-PMN adhesion. Preliminary studies by flow cytometric analysis with isolated platelets and PMNs, using fluorescently-labelled MAbs to P-selectin and GPIIb-IIIa and LDS-751, a nucleic acid stain, as a PMN marker, have been initiated. Our results indicated that PMN isolated from whole blood had platelets adhering to them. When unactivated platelets were coincubated with PMNs, an increase in platelet adherence to PMNs was observed. Activated platelets incubated with PMNs showed a further two- to four- fold increase in the number of platelets bound per PMN.This study will be expanded to characterize the platelet and PMN receptors in heterotypic cell adhesion in isolated cells and in whole blood. Areas of interest include: 1.) Alterations in the cytoskeleton and surface membrane glycoproteins 2.) the potential roles of the a2-integrins (CD11x/CD18) and CD31 in adhesion, and 3.) the role of platelet microvescicles associated with platelet-PMN aggregates.